Electrodialysis treatment

ABSTRACT

A process for reducing the content of organic and inorganic halogen in an aqueous solution of a nitrogen-containing epihalohydrin-based resin, in which process the aqueous resin solution is subjected to an electrodialysis treatment. The aqueous resin solutions obtained by the process are used as additives in the production of paper, board and paper board, in particular as wet-strength agents.

FIELD OF THE INVENTION

The present invention relates to a process for reducing the content oforganic and inorganic halogen in an aqueous solution of anitrogen-containing epihalohydrin-based resin and to the use of theproduct obtained by the process.

BACKGROUND OF THE INVENTION

During recent years attempts to reduce the use of halogen-containingcompounds have gained an increased interest, particularly in the fieldof pulp and paper making. Organic halogen in organic compounds areresponsible for an increased halogen load in waste water as well as inpaper and paper board. Epihalohydrin-based resins are halogen-containingorganic compounds widely used as additives in the production of paper,for instance as wet-strength agents. Many methods have been developedfor reducing the organic halogen content of epihalohydrin-based resins.European patent application 0512423 and U.S. Pat. Nos. 4,857,586 and4,975,499 relate to the treatment of aqueous solutions ofepihalohydrin-based resins with strong bases. European patentapplication 0510987 discloses enzymatic dehalogenation ofhalogen-containing compounds present in aqueous solutions ofepihalohydrin-based resins. However, a major drawback with these methodsis that they only reduce the organic halogen content but increase theinorganic halogen content in the form of halogen ions, whereby the totalhalogen content in the aqueous solution will remain constant. This is aserious limitation since organic halogen will be formed by reactions ofthe halogen ions with organic compounds present in the aqueous solution,in particularly if the pH of the product is lowered to below 7,especially 3-5, for improvement of storage stability.

WO 92/22601 reveals the possibility to remove both organic and inorganichalogen from epihalohydrin-based resins by passing an aqueous solutionthereof through a strongly basic ion-exchange resin. A drawback withthis process is the non-continuous operation which is due to the need toregenerate the ion exchange resin from time to time. Rinsing andregeneration or backwash of the resin also produces effluents, whichstill contain organic compounds causing problems in the waste water dueto their chemical oxygen demand, and rather high salt load sincechemicals for regeneration have to be used in excess.

The use of electrodialysis has been described in the literature onnumerous occasions, see e.g. R. W. Baker et al, Membrane SeparationSystems, Noyes Data Corp., 1991. Electrodialysis is a well establishedtechnique for desalination of brackish water for the production ofpotable water and table salt and it is most frequently used in processesinvolving inorganic material. However, according to U.S. Pat. Nos.4,802,965 and 5,145,569, electrodialysis can also be used for removingsalts from aqueous solutions of organic compounds.

Accordingly, it is an object of the present invention to provide aprocess for treating an aqueous solution of a nitrogen-containingepihalohydrin-based resin in order to produce an aqueous solution of anitrogen-containing epihalohydrin-based resin having a reduced contentof organic and inorganic halogen. It is further an object of theinvention to provide a process as described above which can be carriedout continuously. Another object of the present invention is to providea process as described above which produces an aqueous solution having areduced content of halogenated products and halogenated by-products.Still another object of the invention is to provide a process asdescribed above which reduces the content of organic and inorganichalogen in the aqueous solution of a halogen-containing organic compoundto levels lower than those obtainable by applying known methods.

The objects of the invention are achieved by a process as furtherdefined in the claims. More specifically, the invention relates to aprocess for reducing the content of organic and inorganic halogen in anaqueous solution of a nitrogen-containing epihalohydrin-based resin bysubjecting the aqueous solution to an electrodialysis treatment.

SUMMARY OF THE INVENTION

The present invention generally relates to a process for reducing thehalogen content of aqueous solutions of epihalohydrin-based resins, andto the use of the products obtained by such process. More specifically,the present invention relates to subjecting an aqueous solution of anitrogen-containing epihalohydrin-based resin to an electrodialysistreatment to produce an aqueous solution of a nitrogen-containingepihalohydrin-based resin having a reduced content of organic andinorganic halogen. The aqueous solutions obtained by the process areused as additives in the production of paper.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention it has been found that it is possibleto subject aqueous solutions of nitrogen-containing epihalohydrin-basedresins to electrodialysis without clogging of the membranes. Moreover,it has been unexpectedly discovered that the electrodialytical treatmentof aqueous solutions comprising nitrogen-containing epihalohydrin-basedresins not only removed ionically bound halogen ions but alsosubstantially reduced the content of organic halogen covalently bound toorganic compounds present in the solution. It is believed that epoxidegroups are formed in the epihalohydrin-based resins when the organicbound halogen is removed and that the organic bound halogen is convertedto inorganic halogen.

By electrodialysis is meant any electrochemical process including atleast one ion selective membrane. By organic halogen is meant allhalogen linked to organic molecules. These halogens are preferablylinked by covalent bonds to the organic compound. By inorganic halogenis meant halogen in the form of halogen ions, preferably halide ionssuch as Cl⁻ and Br⁻. The total halogen content is the sum of organichalogen and inorganic halogen.

According to the process of the present invention use can be made of anytype of nitrogen-containing epihalohydrin-based resins. Suitably, theresins are formed by reactions of nitrogen-containing precursorsselected from amines, poly-amines, polyaminoamides and mixtures thereofwith epihalo-hydrins, such as those resins described by Dan Eklund andTom Lindstrom in "Paper Chemistry, An Introduction", page 97, DT PaperScience Publications, 1991. Preferably, the resins arepolyaminoamide-epihalohydrin-based resins, which are also referred to aspolyamidoamine-epihalohydrin-based resins. Epihalohydrins that can beused include epibromohydrin and epichlorohydrin, preferablyepichlorohydrin. Suitably, the resins are produced using 0.5-2.0 molesof epihalohydrin per mole of basic nitrogen in the nitrogen-containingprecursor.

The nitrogen-containing precursor is preferably the polyaminoamidereaction product of a polycarboxylic acid, suitably a dicarboxylic acid,and a polyamine. By the term "carboxylic acid" is meant to includecarboxylic derivatives such as anhydrides, esters and half esters.Suitable poly-carboxylic acids include saturated or unsaturatedaliphatic or aromatic dicarboxylic acids. Preferably, the polycarboxylicacids contain less than 10 carbon atoms.

Suitable polycarboxylic acids include oxalic acid, malonic acid,succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acidand derivatives thereof. Mixture of these compounds can also be applied.Adipic acid is preferred.

Suitable polyamines include polyalkylene polyamines, or mixturesthereof, having the following formula:

    H.sub.2 N--(CR.sup.1 H).sub.a --(CR.sup.2 H).sub.b --N(R.sup.3)--(CR.sup.4 H).sub.c --(CR.sup.5 H).sub.d --NH.sub.2                  (I)

in which R¹ -R⁵ represent hydrogen or lower alkyl, preferably up to C₃and a-d represent integers of from 0 to 4. Preferred polyalkylenepolyamines include diethylene triamine, tri-ethylene tetra amine,tetraethylene penta amine, dipropylene triamine, and mixtures thereof.

The polyamines of formula (I) can be combined with other polyamines ormixtures of other amines. Preferably, these amines have the followingformulae II-VII: ##STR1## in which R⁶ -R¹⁴ represent hydrogen or loweralkyl, preferably up to C₃, e-l represent integers of from 0 to 4, and mrepresents an integer of from 0 to 5.

The polycarboxylic acid and the polyamine can be applied in a mole ratioof from 1:0.5 to 1:1.5.

The nitrogen-containing epihalohydrin-based resin according to theinvention is present in an aqueous solution which may comprise awater-miscible solvent, such as methanol, ethanol or dimethyl formamide.The aqueous resin solution is preferably prepared from an aqueoussolution of a nitrogen-containing precursor. The reaction ofepihalohydrin with a nitrogen-containing precursor can be performed inmany different ways known to the person skilled in the art, such asthose mentioned in WO 92/22601, which is hereby incorporated byreference. The molecular weights of the resins are not critical.Preferably, the molecular weights of the resins are within the range offrom 50 000 to 1 000 000 or even higher.

The production of epihalohydrin-based resins leads to formation ofhalogenated by-products. Aqueous resin solutions produced by reaction ofamines, polyamines or polyaminoamides with epihalohydrin containundesired by-products such as 1,3-dihalo-2-propanol (DXP) and3-halo-1,2-propandiol (XPD). Especially, 1,3-dichloro-2-propanol (DCP)and 3-chloro-1,2-propandiol (CPD) are formed when epichlorohydrin isused. It is encompassed by the process of the invention to reduce thecontent of organic halogen in such low molecular weight organiccompounds as well as oligomeric halogen-containing organic compoundspresent in the resin solutions. By the electrodialysis treatmentaccording to the present invention, DXP and XPD as well as any remainingepihalohydrin can be converted to the halogen-free compounds glycidoland ultimately glycerol.

The solids content of the aqueous solution to be subjected to theelectrodialysis treatment can be as high as 30% by weight or more,preferably 5-25% by weight, most preferably about 15-20% by weight. Theviscosity of the aqueous solutions is preferably within the range of1-100 mPas, most preferably 5-60 mPas. After the electrodialysistreatment the viscosity of the aqueous solution can be raised by furtherpolymerisation of the resin in known manner before the use thereof, forinstance as a wet-strength agent.

Electrodialysis processes and devices are well-known to the skilledperson and electrolysis devices can be made from conventional parts asdescribed, for example, by R. W. Baker et al in "Membrane SeparationSystems", Noyes Data Corp., 1991.

In the process of the present invention, the electrodialysis treatmentcan be conducted in an electrodialysis device containing at least oneelectrodialysis unit cell arranged between an anode and a cathode,wherein the unit cell comprises a compartment which on the side facingtowards the anode is delimited by an anion selective membrane, wherebythe aqueous solution of a nitrogen-containing epihalohydrin-based resinis fed to said compartment and halogen ions are brought to migratethrough said anion selective membrane by establishing an electricalpotential difference between the anode and cathode. The compartment towhich the aqueous resin solution is fed can on the side facing towardsthe cathode be delimited by any membrane preventing transport of thenitrogen-containing epihalohydrin-based resin through said membrane,preferably an anion selective membrane, cation selective membrane orbipolar membrane.

Suitably, non-halogen containing anions are introduced into thecompartment to which the aqueous solution of a nitrogen-containingepihalohydrin-based resin is fed, either through the membrane facingtowards the cathode or included in the aqueous resin solution. Suitablenon-halogen containing anions include hydroxide, sulphate, phosphate,acetate, formiate and mixtures thereof, preferably hydroxide. In theprocess, use is suitably made of an aqueous solution of a salt of thenon-halogen containing anion, the counter-ion of which is not criticalas long as the electrodialysis treatment and equipment are not adverselyaffected. The salt of the non-halogen containing anion should have asolubility in the aqueous solution used sufficient to perform theelectrodialysis treatment and use is suitably made of metal saltsthereof, preferably alkali metals. Examples of suitable metal salts ofnon-halogen containing anions include LiOH, NaOH, KOH, Na₃ PO₄,Na-acetate and Na-formiate. NaOH and KOH are preferably used. Theaqueous solution used can have a concentration of the salt ofnon-halogen containing anion of from 0.001M or less up to saturation ofthe solution used, preferably 0.05-10M, most preferably 0.1-5M.

In a preferred embodiment of the present invention, the electrodialysisunit cell comprises first and second compartments and first and secondanion selective membranes. The first anion selective membrane is facingtowards the cathode, the second anion selective membrane is facingtowards the anode, the first compartment is facing towards the cathodeand is delimited by the first anion selective membrane, and the secondcompartment is delimited by the first and second anion selectivemembranes. In the process, the aqueous solution of a nitrogen-containingepihalohydrin-based resin is fed to the second compartment, non-halogencontaining anions are fed to the first compartment and brought tomigrate through the first anion selective membrane, and halogen ions arebrought to migrate through the second anion selective membrane. Adjacentto the unit cell and facing towards the anode there can be an anodecompartment.

According to further preferred embodiments of the invention, theelectrodialysis unit cell comprising first and second compartments andfirst and second anion selective membranes further comprises a thirdcompartment and a cation selective or bipolar membrane facing towardsthe anode. In the process, the halogen ions are brought to migrate intothe third compartment delimited by the second anion selective membraneand the cation selective or bipolar membrane. The feed to the thirdcompartment is suitably an aqueous solution of a salt or metal halidewhen the membrane of the third compartment facing towards the anode is acation selective membrane, and suitably water or aqueous hydrochloricacid when the membrane of the third compartment facing towards the anodeis a bipolar membrane. Adjacent to the unit cell and facing towards theanode there can be an anode compartment.

In another preferred embodiment of the invention, the electrodialysisunit cell comprises first and second compartments, an anion selectivemembrane and a bipolar membrane, wherein the bipolar membrane is facingtowards the cathode, the anion selective membrane is facing towards theanode, the first compartment is facing towards the cathode and isdelimited by the bipolar membrane, and the second compartment isdelimited by the bipolar membrane and anion selective membrane. In theprocess, the aqueous solution of a nitrogen-containingepihalohydrin-based resin is fed to the second compartment, an aqueoussolution of an acid or salt is fed to the first compartment and halogenions are brought to migrate through the anion selective membrane.Alternatively, water can be fed to the first compartment. Adjacent tothe unit cell and facing towards the anode there can be an anodecompartment.

In another preferred embodiment of the invention, the electrodialysisunit cell comprising first and second compartments, an anion selectivemembrane and a bipolar membrane--facing towards the cathode furthercomprises a third compartment and a cation selective membrane facingtowards the anode. In this process, the halogen ions are brought tomigrate into the third compartment delimited by the anion selectivemembrane and the cation selective membrane. The feed to the thirdcompartment can be an aqueous solution of a salt, metal halide or acid.Adjacent to the unit cell and facing towards the anode there can be ananode compartment.

Aqueous solutions of salts, metal halides and acids that can be used inthe process of the invention are not critical as long as theelectrodialysis treatment and equipment are not adversely affected bythe solutions. Suitable salts include salts having good conductivity,such as salts of strong bases and strong acids, e.g. NaCl, KCl, LiCl,Na₂ SO₄, K₂ O₄, Li₂ SO₄, NaNO₃, NH₄ Cl and R₄ NCl. Preferably, the saltis electrochemically inert. The salt concentration in the aqueoussolutions used can be from 0.001M up to saturation of the solution,preferably 0.1-5M. As examples of suitable metal halides can bementioned alkali metal halides, e.g. LiCl, LiBr, NaCl, NaBr, KCl, andKBr. NaCl and KCl are preferably used. The metal halide concentration inthe aqueous solutions used can be of from 0.001M up to saturation of thesolution, preferably 0.1-5.0M. Suitable acids include organic andinorganic acids and mixtures thereof, preferably inorganic acids. Asexamples of suitable inorganic acids can be mentioned hydrochloric acid,sulphuric acid, nitric acid and phosphoric acid, and use is preferablymade of hydrochloric and sulphuric acid. The acid concentration in theaqueous solutions used can be from 0.001M up to 10M or even higher,preferably 0.1-5M.

In accordance with another preferred embodiment of the invention, theaqueous solution of a nitrogen-containing epihalohydrin-based resin ispre-treated with hydroxide ions before the electrodialysis treatment,whereby a part of the organic halogen in the resin is replaced byhydroxide and the following electrodialysis treatment can be carried outin commercially available equipment for desalination of water. It wassurprisingly found that this pre-treatment did not cause unduepolymerisation of the resin in solution or onto the membranes. Thepre-treatment can be carried out by adding a hydroxide-containing saltor an aqueous solution thereof to the resin solution. Use is suitablymade of metal hydroxides or mixtures of metal hydroxides, preferably analkali metal hydroxide. Examples of suitable metal hydroxides includeLiOH, NaOH and KOH. NaOH and KOH are preferably used. The pH of theaqueous resin solution after the pre-treatment is suitably above 5,preferably 8-13.

According to another preferred embodiment of the present inventioninvolving hydroxide pre-treatment of the resin solution, theelectrodialysis unit cell can comprise first and second compartments, ananion selective membrane and a first cation selective membrane. Theanion selective membrane is facing towards the anode, the first cationselective membrane is facing towards the cathode, the first compartmentis facing towards the cathode and is delimited by the first cationSelective membrane, and the second compartment is delimited by the firstcation selective membrane and anion selective membrane. In the process,the aqueous solution of a nitrogen-containing epihalohydrin-based resinand hydroxide ions, e.g. in the form of a hydroxide-containing salt, arefed to the second compartment and halogen ions are brought to migratethrough the anion selective membrane. The counter-ions to the hydroxideions are brought to migrate through the first cation selective membraneinto the first compartment. Adjacent to the unit cell and facing towardsthe anode there can be an anode compartment through which an aqueoussolution of a metal halide or acid can be passed. Water or preferably anaqueous solution of a metal hydroxide or metal halide is fed to thefirst compartment.

In accordance with another preferred embodiment of the present inventioninvolving pre-treatment of the resin solution, the electrodialysis unitcell comprising first and second compartments, an anion selectivemembrane and first cation selective membrane further comprises a thirdcompartment and a second cation selective membrane facing towards theanode, whereby the halogen ions are brought to migrate into the thirdcompartment delimited by the anion selective membrane and second cationselective membrane. An aqueous solution of a salt or metal halide aspreviously described can be fed to the third compartment and an aqueousmetal hydroxide solution can be fed to the first compartment. Adjacentto the unit cell and facing towards the anode there can be an anodecompartment, through which an aqueous metal hydroxide solution can bepassed.

The anion selective membranes, also referred to as anion-exchangemembranes, used according to the present invention permit exchange ofanions between compartments delimited by such an anion selectivemembrane. Examples of suitable anion selective membranes are those soldunder the tradename Neosepta (manufactured by Tokuyama Soda). The cationselective membranes, also referred to as cation-exchange membranes,permit exchange of cations between compartments delimited by such ancation selective membrane. Examples of suitable cation selectivemembranes to be used in the process of the invention are those soldunder the tradename Nafion (manufactured by DuPont). The bipolarmembranes permit electrically forced dissociation of water and suitablebipolar membranes include those sold and manufactured by WSI. Thecompartments in the electrodialysis devices defined by the gaps betweenmembranes and the gaps between membranes and electrodes are equippedwith inlets and outlets for the flow-through of solutions.

Preferably, the current densities in the process of the presentinvention are within the range of 0.01-5 kA/m², most preferably withinthe range of 0.1-1 kA/m².

The temperature of the aqueous solutions fed to the compartments shouldbe adapted to the membranes and to the resin solution used. Chemicalreactions, e.g. polymerisation, may take place if the temperature is toohigh and, therefore, the temperature is preferably low so as to avoidundue polymerisation of the epihalohydrin-based resin. Suitably, thesolutions are cooled in order to balance the temperature increase due tothe electrodialysis treatment. The temperature may be within the rangeof from the freezing point of the aqueous solutions to about 40° C.,preferably below 20° C. and most preferably between 5° and 20° C.

According to a preferred embodiment of the invention, the aqueoussolution of a nitrogen-containing epihalohydrin-based resin is alsocontacted with an anion-exchange resin, which can be carried out before,simultaneously with or after the electrodialysis treatment, preferablysimultaneously with or after the electrodialysis treatment. Suitably,when the simultaneous mode of operation is applied, the compartment towhich the aqueous resin solution is fed contains the anion-exchangeresin. Anion-exchange resins are known to the skilled person andreference is made to Ullmann's Encyclopedia of Industrial Chemistry,Vol. A14, page 393 ff, 1989. Suitably, a basic anion-exchange resin isused which generally carry cationic groups such as R--NH₃ ⁺, R₂ NH₂ ⁺,R₃ NH⁺, R₄ N⁺ and R₃ S⁺, in which at least one R in each of thementioned groups represent the polymer matrix. Examples of polymermatrices that can be used include those based on polystyrene,polyacrylic, phenol-formaldehyde, and polyalkylamine resins.Anion-exchange resins that can be used in the process of the presentinvention are described by Ullmann in the above edition and in WO92/22601, which is incorporated herein by reference.

Preferably, the basic anion-exchange resin used in the process of thepresent invention contains tertiary amino groups or quaternary ammoniumgroups or mixtures thereof. Strongly basic anion-exchange resins arepreferred over weakly basic anion-exchange resins. Examples of stronglybasic anion-exchange resins include resins carrying quaternary ammoniumgroups having three lower alkyl substituents or quaternary ammoniumgroups containing at least one lower alcohol substituent. Mixed resinscan also be used. The most preferred anion-exchange resins are stronglybasic anion-exchange resins of the type carrying quaternary ammoniumsubstituents selected from the group consisting of trimethyl ammonium,dimethyl-ethanol ammonium, and mixtures thereof.

The aqueous solution of a nitrogen-containing epihalohydrin-based resincan have a low pH before it is subjected to the electrodialysistreatment, e.g. a pH of about 4 or even lower. During the treatment thepH is often raised to high values, e.g. to a pH of about 12 or evenhigher. Suitably, the pH of the aqueous resin solution is adjusted withacid after the treatment to provide a product having a pH of lower than5. Preferably, the pH is adjusted to a value of about 3-5 to obtain anaqueous resin solution having better stability upon storage. The pH canbe adjusted by employing any feasible organic or inorganic acid or anymixture thereof. Preferred organic acids include formic, acetic andcitric acid, whereas preferred inorganic acids include sulphuric acidand phosphoric acid.

In another preferred embodiment of the invention, the polarities of theelectrodes are switched over at least once during the process, suitablyat regular intervals. This may be applied in order to minimize anyfouling on the membranes delimiting the compartment to which the aqueousresin solution is fed and preferably when use is made of unit cells notcontaining bipolar membranes. Suitably, the feeds to the compartmentsadjacent to the compartment to which the aqueous resin solution is fedare also switched over at least once, preferably at the same intervalsas the electrode polarities, so as to avoid introduction of halogen ionsinto the aqueous resin solution. If need be, further feeds may beswitched over, as will be easily appreciated by the skilled person.

According to another preferred embodiment of the invention, theelectrodialysis device contains at least two electrodialysis unit cells.Suitably, the device comprises a row of adjacent unit cells arranged inthe form of a stack between the anode and cathode. A multi unit celldevice can contain unit cells of the same type or unit cells ofdifferent types. It will be appreciated by the person skilled in the artwhich unit cells that are preferably stacked. In a multi unit celldevice, the discharge stream from one cell compartment can be the feedstream of another cell compartment.

In the present process, the anode can be made of any electricallyconducting material stable under anodic polarisation in the anolytesolution. Use can be made of dimensionally stable anodes, which can bemade of titanium, zirconium, hafnium, niobium or mixtures thereof,having an active surface layer of ruthenium, iridium, platinum,palladium or mixtures thereof. Examples of suitable commercial anodesare those sold by Permascand under the name DSA. Suitable anodes canalso be made of graphite.

Typically, the anode reaction is oxygen evolution according to thefollowing reaction:

    H.sub.2 O→1/2O.sub.2 +2H.sup.+ +2e.sup.-

If halogen ions are present in the anolyte, halogen formation will takeplace at the anode. Thus, if chloride ions are present in the anolyte,chlorine formation takes place according to the following reaction:

    2Cl.sup.- →Cl.sub.2 +2e.sup.-

The anode can also be a hydrogen depolarised anode where hydrogen gas isoxidised in a gas diffusion electrode according to the followingreaction:

    H.sub.2 →2H.sup.+ +2e.sup.-

The cathode is suitably made of an electrically con-ducting materialstable under cathodic polarisation in the catholyte. As examples ofcathode materials can be mentioned steel, stainless steel, nickel andgraphite. The cathode can also be coated with various catalysts, e.g.ruthenium oxides. Typically, the cathode reaction is hydrogen evolutionaccording to the following reaction:

    2e.sup.- +2H.sub.2 O→H.sub.2 +2OH.sup.-

The cathode can also be an oxygen depolarised cathode where oxygen isreduced in a gas diffusion electrode according to the followingreaction:

    1/2O.sub.2 +H.sub.2 O+2e.sup.- →2OH.sup.-

The electrodialytical treatment of the invention can be performed as abatch, semi-continuous or continuous process. Preferably, asemi-continuous or continuous process is used, most preferably acontinuous process. The continuous process comprises continuouslyfeeding the aqueous solution of a nitrogen-containingepihalohydrin-based resin into a cell compartment, continuouslysubjecting the resin solution to the electrodialysis treatment followedby continuously withdrawing the solution from the compartment. The resinsolution can be recirculated and it is suitably recirculated until thedesired content of organic and inorganic halogen is obtained.

Flow rates that are feasible according to the invention are dependent onthe process conditions and are easily determined by the person skilledin the art taking into consideration factors such as the electrodialysisdevice used, size of the compartments, production capacity and currentdensities.

The present invention also relates to the use of the aqueous solution ofa nitrogen-containing epihalohydrin-based resin having a reduced contentof organic and inorganic halo-gen obtained by the process as an additivein the production of paper, board and paper board. The aqueous resinsolution is preferably used as a wet-strength agent but it can also beused as a retention aid, anionic trash catcher and sizing promotor.

The invention will now be described in more detail with reference to theaccompanying drawings 1-5. However, the invention is not restricted tothe embodiments illustrated, but many other variants are feasible withinthe scope of the claims. The solutions mentioned below are aqueoussolutions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an electrodialysis devicecontaining one electrodialysis unit cell comprising two anion selectivemembranes.

FIG. 2 illustrates the electrodialysis device of FIG. 1 furthercomprising one cation selective membrane.

FIG. 3 is a schematic view illustrating an electrodialysis devicecontaining two electrodialysis unit cells of FIG. 2.

FIG. 4 is a schematic view illustrating an electrodialysis devicecomprising one anion selective membrane, one cation selective membraneand one bipolar membrane.

FIG. 5 is a schematic view illustrating an electrodialysis devicecontaining two different electrodialysis unit cells suitably used whenthe aqueous resin solution is pretreated with hydroxide ions.

FIG. 1 schematically illustrates an electrodialysis device (1)containing one electrodialysis unit cell (2) arranged between an anode(A) and a cathode (C). The unit cell comprises first and secondcompartments (3,4) and first and second anion selective membranes (5,6).A further compartment adjacent to the unit cell is facing towards theanode, here-after named the anode compartment (7). An aqueous solutioncomprising a nitrogen-containing epihalohydrin-based resin is passedthrough the second compartment (4), a sodium hydroxide solution ispassed through the first compartment (3) and a solution of sodiumchloride or sulphate is passed through the anode compartment (7).

By establishing an electrical potential difference between theelectrodes, hydroxide ions present in the first compartment (3) arebrought to migrate through the first anion selective membrane (5) intothe second compartment (4) and organic and inorganic halogen present inthe resin are brought to migrate as halogen ions through the secondanion selective membrane (6) into the anode compartment (7). As a resultof the electrodialysis treatment, an aqueous solution comprising anitrogen-containing epihalohydrin-based resin having a reduced contentof organic and inorganic halogen is withdrawn from the secondcompartment.

The feed to the anode compartments in the process of the invention canbe an aqueous solution of a salt, metal halide, acid or metal hydroxideas previously defined. The ions should have good conductivity in thesolution and they are suitably electrochemically inert.

FIG. 2 illustrates an electrodialysis device (8) similar to the deviceas outlined in FIG. 1, in which the unit cell (9) further comprises athird compartment (10) and a cation selective membrane (11) facingtowards the anode. A further compartment adjacent to thiselectrodialysis unit cell is facing towards the anode, hereafter namedthe anode compartment (12). Solutions are passed through the first andsecond compartments (3,4) as described above. In addition, a sodiumchloride solution is passed through the third compart-ment (10) and asodium hydroxide solution is passed through the anode compartment (12).

By applying an electrical potential difference between the electrodes,hydroxide ions present in the first compart-ment (3) are brought tomigrate through the first anion selective membrane (5), halogen ionspresent in the second compartment (4) are brought to migrate through thesecond anion selective membrane (6) and sodium ions present in the anodecompartment (12) are forced to migrate through the cation selectivemembrane (11) into the third compartment (10), thereby to combine withthe halogen ions entering from the second compartment (4) and forming afortified sodium halide solution in the third compartment (10). By theelectrodialysis treatment the content of organic and inorganic halogenis reduced in the aqueous resin solution. A hydroxide feed stream can bedivided into feed solutions to the first compartment and to the anodecompartment, respectively, and the discharge solutions from saidcompartments can be brought together to one stream which may berecirculated.

The electrodialysis device can contain two or more unit cells. FIG. 3shows a device (13) containing two unit cells of the type as outlined inFIG. 2 between the anode (A) and cathode (C). An anode compartment (14)is arranged between the anode and the unit cell facing towards theanode. The solutions are preferably recirculated, either back to thecompartments from which they originated or to a correspondingcompartment of another cell.

When a bipolar membrane is used in the process of the present invention,the electrodialysis device can be designed as outlined in FIG. 4. In thedevice (15), the unit cell (16) comprises first, second and thirdcompartments (17,18,19), a bipolar membrane (20), an anion selectivemembrane (21) and a cation selective membrane (22). A furthercompartment adjacent to this electrodialysis unit cell is facing towardsthe anode, hereafter named the anode compartment (23). An aqueoussolution comprising a nitrogen-containing epihalohydrin-based resin isfed to the second compartment (18), an aqueous sulphuric acid solutionis passed through the first compartment (17) and to the anodecompartment (23), respectively, and water or a hydrochloric acidsolution is passed through the third compartment (19).

By establishing an electrical potential difference between theelectrodes, the electrically forced dissociation of water in the bipolarmembrane (20) results in the transfer of hydroxide ions into the secondcompartment (18). In addition, halogen ions present in the secondcompartment are brought to migrate through the anion selective membrane(21) into the third compartment (19) and protons fed to the anodecompartment (23) are brought to migrate through the cation selectivemembrane (22) into the third compartment (19), in which a fortifiedsolution of hydrohalide acid is formed.

A bipolar membrane multi unit cell device suitably contains at leastone, preferably more than one unit cell of the type comprising an anionselective membrane and a bipolar membrane. It is preferred that a numberof such cells are stacked between the electrodes and are facing towardsthe cathode. Preferably, this device further contains a unit cell of thetype as described in FIG. 4 facing towards the anode.

FIG. 5 is a schematically view illustrating an electrodialysis devicethat can be used to reduce the content of organic and inorganic halogenin aqueous resin solutions pretreated with hydroxide ions. The device(24) contains two different electrodialysis unit cells, wherein thefirst unit cell (25) is facing towards the cathode and comprises firstand second compartments (26,27), a first cation selective membrane (28)and an anion selective membrane (29), and the second unit cell (30)comprises first, second and third compartments (31,32,33), a firstcation selective membrane (34), an anion selective membrane (35) and asecond cation selective membrane (36). A further compartment adjacent tothe second electrodialysis unit cell is facing towards the anode,hereafter named the anode compartment (37). Aqueous solutions comprisinga nitrogen-containing epihalohydrin-based resin and sodium hydroxide arepassed through the second compartments (27,32) of both unit cells, anaqueous sodium hydroxide solution is passed through the firstcompartment (26) of the first unit cell, an aqueous sodium chloridesolution is passed through the first compartment (31) of the second unitcell, an aqueous hydrochloric acid solution is passed through the thirdcompartment (33) and an aqueous sulphuric acid solution is passedthrough the anode compartment (37).

By applying an electrical potential difference between the electrodes,halogen ions present in the resin solutions are brought to migratethrough the anion selective membranes (29,35) of both unit cells intothe first and third compart-ments (31,33) of the second unit cell,respectively, and sodium ions present in the resin solutions are broughtto migrate through the first cation selective membranes (28,34) of bothunit cells into the first compartments (26,31) of both unit cells,respectively. The resin solutions can be recircu-lated and furthersodium hydroxide can be added to the resin solutions during the process.

A multi unit cell device for the treatment of aqueous resin solutionspre-treated with hydroxide ions as outlined in FIG. 5 suitably containsat least one, preferably more than one unit cell of the type comprisinga cation selective membrane and an anion selective membrane. It ispreferred that a number of such cells are stacked between the electrodesand are facing towards the cathode. Preferably this device furthercontains a unit cell facing towards the anode which is of the typecomprising a first cation selective membrane, an anion selectivemembrane and a second cation selective membrane.

The invention is further illustrated by the following examples which,however, are not intended to limit the scope of the invention. Parts andper cent relate to parts by weight and per cent by weight respectively,unless otherwise stated. The solutions used in the examples are aqueoussolutions.

Example 1: An electrodialysis device of the type as essentially outlinedin FIG. 2 was used for the electrodialysis treatment of apolyaminoamide-epichlorohydrin-based resin manufactured as described inExample 3 of WO 92/22601. The resin solution had a solids content of 20%by weight, a viscosity of 12 mPas and the treatment was started at atemperature of 20° C.

Approximately 2 l of an initially 1M sodium hydroxide solution and 2 lof an initially 0.1M sodium chloride solution were passed through thecompartments as described in FIG. 2. The process was performed bycontinously pumping the solutions through the compartments with a flowrate of 140 ml/h and passing an electrical current of 10 A through thecompartments. The initial voltage was 6.9 V. The electro-dialysis devicehad an electrode surface area of 250 cm², and hence the current densityamounted to 40 mA/cm².

After 100 min the treatment was stopped and the collected resin solutionwas heated 35° C. and kept at this temperature until a viscosity of 20mPas (25° C.) was reached. The pH of the resin solution was adjusted to3.5 by addition of sulphuric acid.

The analytical data of the resin solution were as follows:

    ______________________________________                                                        Before After                                                                  treatment                                                                            treatment                                              ______________________________________                                        Organic chlorine (OX)                                                                           0.45%    290      ppm                                       Inorganic chlorine (Cl.sup.-)                                                                   2.10%    170      ppm                                       Total chlorine    2.55%    460      ppm                                       DCP content       1250 ppm <8       ppm                                       CPD content        260 ppm <8       ppm                                       AOX                 3.8 g/l                                                                              25       ppm                                       ______________________________________                                    

The content of total chlorine was determined using an AOX-combustionapparatus according to a standard method. The content of inorganicchlorine was determined by using argento-metric titration. The contentof organic chlorine was calculated as the difference between content oftotal chlorine and inorganic chlorine. The contents of DCP and CPD weredetermined by using a gas chromatographic method having a detectionlimit of 8 ppm. The AOX (absorbable organic halogen) was determined inaccordance with DIN 38049, part 14.

As is evident, a considerable reduction of the contents of organic andinorganic chlorine as well as by-products was achieved.

Example 2: In this example the electrodialysis device of Example 1 wasused, with the difference that the space between the two anion selectivemembranes, through which the resin solution is pumped, was filled with astrongly basic anion-exchange resin (Levatit™ M206, manufactured byBayer).

The solutions of epihalohydrin-based resin, NaOH and NaCl as initiallyused in the process of Example 1 were similarily used in this example.The solutions were pumped through the compartments with a flow rate of190 ml/h while passing an electrical current of 10 A between theelectrodes. The voltage was about 7.0 to 8.0 V.

After 3 h the treatment was stopped and the collected resin solution washeated to 30° C. for further polymerisation until a viscosity of about20 mPas was reached. Then the pH was adjusted with sulphuric acid to3.6.

The analytical data of the resin solution were as follows:

    ______________________________________                                                        Before After                                                                  treatment                                                                            treatment                                              ______________________________________                                        Organic chlorine (OX)                                                                           0.45%    110      ppm                                       Inorganic chlorine (Cl.sup.-)                                                                   2.10%    120      ppm                                       Total chlorine    2.55%    230      ppm                                       DCP content       1250 ppm <8       ppm                                       CPD content        260 ppm <8       ppm                                       AOX                 3.8 g/l                                                                              <20      ppm                                       ______________________________________                                    

The analytical data were determined as described in Example 1.

Example 3: The electrodialytical device of Example 1 was used in thisexample, with the difference that the first anion selective membranefacing towards the cathode was replaced by a cation selective membrane.A polyaminoamide-epichlorohydrin-based resin was prepared in a mannersimilar to that described in Example 3 of WO 92/22601, but using a molarratio of epichlorohydrin which was increased by 5%. The resin solutionhad a solids content of 19% by weight, a pH of 5 and a viscosity of 19mPas.

The resin solution was pre-treated by adding a sodium hydroxidesolution, prepared from 20 ml 50% NaOH and 85 ml of water, to 395 g ofthe resin solution at room temperature. The resulting resin solution hada solids content of 15% by weight. The alkaline pre-treated resinsolution was placed in a beaker cooled with an ice-bath and continuouslypumped through the second compartment with a flow rate of 5 l/h. Inaddition, an initially 1M sodium hydroxide solution was continuouslypumped through the first and anode compartments, respectively, and aninitially 0.1M sodium chloride solution continuously pumped through thethird compartment. The initial electrical current and voltage was 10.0 Aand 9.5 V, respectively.

After 3 h 5 ml of 50% NaOH solution was further added to the pre-treatedresin solution. After 41/4 h the process was stopped. The alkaline resinsolution (pH≈13) was heated to 40° C. and kept at this temperature untila viscosity of 20 mPas was reached. The pH was adjusted with sulphuricacid to 3.6. The product had a solids content of 17.7% by weight.

The analytical data of the resin solution were as follows:

    ______________________________________                                                     Before pretreatment and                                                                     After                                                           electrodialysis treatment                                                                   treatment                                          ______________________________________                                        Organic chlorine (OX)                                                                        0.95%           300    ppm                                     Inorganic chlorine (Cl.sup.-)                                                                1.74%           280    ppm                                     Total chlorine 2.69%           580    ppm                                     DCP content    3416 ppm        <10    ppm                                     CPD content     906 ppm        20     ppm                                     AOX              4.6 g/l       47     ppm                                     ______________________________________                                    

The analytical data were determined as described in Example 1.

Example 4: An electrodialysis device of the type as essentially outlinedin FIG. 4 comprising a bipolar membrane was used for theelectrodialytical treatment of the polyaminoamide-epichlorohydrin-basedresin solution prepared as described in Example 3. 395 g of the resinsolution was diluted with 105 ml of water to yield a solids content of15% by weight. The resin solutions was cooled with an ice-bath andcontinuously pumped through the second compartment with a flow-rate of7.5 l/h. In addition, an initially 1M sulphuric acid solution and waterwere continuously pumped through the compartments as described in FIG.4. In the process the electrical current and voltage amounted to 5.0 Aand 18-30 V, respectively.

After 1 h 50 min, the electrodialytical treatment was stopped and theresin solution was heated to 30° C. and kept at this temperature until aviscosity of 20 mPas was reached. The pH was adjusted to 3.5 by additionof sulphuric acid.

The analytical data of the resin solution were as follows:

    ______________________________________                                                        Before After                                                                  treatment                                                                            treatment                                              ______________________________________                                        Organic chlorine (OX)                                                                           0.95%      670    ppm                                       Inorganic chlorine (Cl.sup.-)                                                                   1.74%      880    ppm                                       Total chlorine    2.69%      1550   ppm                                       DCP content       3416 ppm   15     ppm                                       CPD content        906 ppm   57     ppm                                       AOX                 4.6 g/l  76     ppm                                       ______________________________________                                    

The analytical data were determined as described in Example 1.

Example 5: In this example, the wet-strength efficiency of the resinsolutions prepared in Examples 1-4 was tested. Test sheets ofapproximately 70 g/m² were prepared on a pilot paper machine (speed 2m/min), capacity 2 kg/h). The furnish consisted of a 30/35/35 blend ofbleached pine sulphate/birch sulphate/beech sulphate which had beenbeaten to a Schopper-Riegler freeness of 26° SR. The fillers DX 40(Omua) and clay (Kaolin B), each in 5% by weight, were added to thestock at a temperature of 25° C. The resin solutions were fed to thepaper machine after the stock dilution. The stock consistency at theheadbox amounted to 0.3% and pH remained in the range of 7.2-7.8 for allproducts and concentrations, and were not adjusted. The temperatures ofthe cylinders in the drying section were adjusted to 60° C./80° C./90°C./110° C.

The paper was cured for 30 min at 100° C. for 2 h before testing. Paperstrips were immersed in distilled water for 5 min at 23° C. beforebreaking length determinations using an Alwetron TH1™ hydrodynamictester (Gockel & Co. GmbH, Munich).

The test results were as follows:

    ______________________________________                                        Dosage                                                                        (% on    Breaking length wet (m)                                              dry content)                                                                           Example 1 Example 2 Example 3                                                                             Example 4                                ______________________________________                                        0.3      730       730       840     810                                      0.6      980       990       1120    1140                                     0.9      1180      1140      1295    1280                                     ______________________________________                                    

I claim:
 1. A process for reducing the content of organic and inorganicchlorine in an aqueous solution containing apolyaminoamide-epichlorohydrin-based resin, which comprises subjectingsaid aqueous solution to an electrodialysis treatment.
 2. The process ofclaim 1, wherein said electrodialysis treatment is conducted in anelectrodialysis device containing at least one electrodialysis unit cellarranged between an anode and a cathode, wherein the unit cell comprisesa compartment which on the side facing towards the anode is delimited byan anion selective membrane, whereby the aqueous solution containing thepolyaminoamide-epichlorohydrin-based resin is fed to said compartmentand halogen ions are brought to migrate through said anion selectivemembrane by establishing an electrical potential difference between theanode and cathode.
 3. The process of claim 2, wherein theelectrodialysis unit cell (2) comprises first and second compartments(3,4) and first and second anion selective membranes (5,6), wherein thefirst anion selective membrane (5) is facing towards the cathode (C),the first compartment (3) is facing towards the cathode and is delimitedby the first anion selective membrane (5), the second compartment (4) isdelimited by the first and second anion selective membranes (5,6),whereby the aqueous solution containing thepolyaminoamide-epichlorohydrin-based resin is fed to the secondcompartment (4), non-halogen containing anions are fed to the firstcompartment (3) and halogen ions are brought to migrate through thesecond anion selective membrane (6).
 4. The process of claim 3, whereinsaid electrodialysis unit cell (9) further comprises a third compartment(10) and a cation selective membrane (11) facing towards the anode (A),whereby the halogen ions are brought to migrate into the thirdcompartment (10) delimited by the second anion selective membrane (6)and cation selective membrane (11).
 5. The process of claim 3, whereinsaid electrodialysis unit cell further comprises a third compartment anda bipolar membrane facing towards the anode, whereby the halogen ionsare brought to migrate into the third compartment delimited by thesecond anion selective membrane and bipolar membrane.
 6. The process ofclaim 2, wherein the electrodialysis unit cell comprises first andsecond compartments, an anion selective membrane and a bipolar membrane,wherein the bipolar membrane is facing towards the cathode, the firstcompartment is facing towards the cathode and is delimited by thebipolar membrane, the second compartment is delimited by the bipolarmembrane and anion selective membrane, whereby the aqueous solutioncontaining the polyaminoamide-epichlorohydrin-based resin is fed to thesecond compartment, an aqueous solution of an acid or salt is fed to thefirst compartment and halogen ions are brought to migrate through theanion selective membrane.
 7. The process of claim 6, wherein theelectrodialysis unit cell (16) further comprises a third compartment(19) and a cation selective membrane (22) facing towards the anode (A),whereby the halogen ions are brought to migrate into the thirdcompartment (19) delimited by the anion selective membrane (21) andcation selective membrane (22).
 8. The process of claim 2, wherein theelectrodialysis unit cell (25) comprises first and second compartments(26,27), an anion selective membrane (29) and a first cation selectivemembrane (28), wherein the first cation selective membrane (28) isfacing towards the cathode (C), the first compartment (26) is facingtowards the cathode and delimited by the first cation selective membrane(28), the second compartment (27) is delimited by the first cationselective membrane (28) and anion selective membrane (29), whereby theaqueous solution containing the polyaminoamide-epichlorohydrin-basedresin and hydroxide ions are fed to the second compartment (27) andhalogen ions are brought to migrate through the anion selective membrane(29).
 9. The process of claim 8, wherein the electrodialysis unit cell(30) further comprises a third compartment (33) and a second cationselective membrane (36) facing towards the anode (A), whereby thehalogen ions are brought to migrate into the third compartment (33)delimited by the anion selective membrane (35) and second cationselective membrane (36).
 10. The process of claim 2, wherein the aqueoussolution containing the polyaminoamide-epichlorohydrin-based resin iscontacted with an anion-exchange resin before, simultaneous with orafter the electrodialysis treatment.
 11. The process of claim 10,wherein the anion-exchange resin is a strongly basic anion-exchangeresin.
 12. The process of claim 2, wherein the electrodialysis devicecontains at least two electrodialysis unit cells.
 13. The process ofclaim 2 wherein the electrodialysis treatment is conducted continuously.14. The process of claim 1, wherein thepolyaminoamide-epichlorohydrin-based resin has a molecular weight of atleast about 50,000.
 15. The process of claim 2, wherein thepolyaminoamide-epichlorohydrin-based resin has a molecular weight of atleast about 50,000.
 16. The process of claim 1 wherein the aqueoussolution contains from about 5% to about 25% by weight ofpolyaminoamide-epichlorohydrin-based resin.
 17. The process of claim 2wherein the aqueous solution contains from about 5% to about 25% byweight of polyaminoamide-epichlorohydrin-based resin.